In drilling an oil well, a drill string having a drill bit affixed to the bottom is customarily used. The drill string is connected into a mud flow system which typically utilizes drilling mud which is pumped by a multiple cylinder pump. The pump connects through a mud line to the top of the drill string, and the mud is delivered under pressure to the top of the drill string for flowing through the drill string to the drill bit. The pump typically operates at high pressures, and pressures in the range of 2,000 to 3,000 psi at the pump discharge outlet are not uncommon. Typical pumps are multiple cylinder pumps. During the operation of the pumps, there are pressure surges in the range of 200 to 300 psi which are caused by power strokes of individual pistons within the pump. These pressure surges are quite large, especially at the surface where there is a minimum of pressure surge damping in the mud delivery system.
Measuring while drilling apparatus has been known heretofore. Such equipment typically operates by forming variable constrictions in the drill string. This forms a pressure pulse transferred through the standing column of mud in the drill string back to the surface. As an example, a constriction might be formed by the measuring while drilling apparatus which signal is then coupled through 10,000 feet of mud standing as a column within the drill pipe. While the mud might be deemed to be an incompressible fluid, nevertheless, the signal received at the surface from the downhole equipment is relatively small. It is relatively small, smaller than the pump surges found at the surface. The pressure pulses or variations at the surface are large; they can be ten to one hundred times greater than the variable data from the measuring while drilling apparatus.
In the operation of measuring while drilling apparatus, pressure pulses travel through the mud at a velocity equal to the acoustic velocity of the medium. Depending on the makeup of the drilling mud, this is a velocity of about 4,000 to 5,000 feet per second. Moreover, each pressure pulse is accompanied by a change in fluid velocity which is defined by the relationship quantifying the water hammer effect. The pressure change-velocity change relationship is therefore given by the following equation: EQU .DELTA.P=R.sub.0 C.DELTA.V (6)
where .DELTA.P=the magnitude of the pressure pulse,
R.sub.0 =the fluid mass density, PA1 C=the acoustic velocity in the fluid, and PA1 .DELTA.V=the change in fluid velocity.
As will be understood from the foregoing equation, pressure pulses formed by measuring while drilling apparatus are related to fluid velocity changes in the foregoing equation.
A typical mud pump forms a pressure surge during the power stroke of the individual pistons in it. This represents a positive pressure surge. This increases the mud flow velocity in the drill string. Conversely, pressure pulses from measuring while drilling apparatus located downhole decrease the mud flow velocity as a result of propogation in the opposite direction. Taking into account the direction of propogation in the system, there is, therefore, an interesting relationship. For a given pressure surge originating with the pump and moving downstream in the same direction as the mud flow, there is a positive pressure increase and a related velocity increase. Conversely, where the pressure pulse is originated at the downhole equipment and moves upstream against the flow of mud, a pulse originating at measuring while drilling apparatus and propogated upstream against the flow of mud is accompanied by a decrease in velocity of mud in the drill string. Intuitively, this conforms to the observation that measuring while drilling apparatus which momentarily constricts the mud flow to form a pressure pulse also retards the mud flow velocity.
This is a linear phenomena, and, thus, pulses traveling in both directions add algebraically. Pressure and velocity variations are thus cumulative.